System for signaling by electromagnetic waves



May 26, 1942; a 2,284,266

H. J. J. M. DE REGNAUL D DE BELLESCIZEv SQYSTEM FOR SIGNALING BY ELECTQOMAGNETIC WAVES Filed April 7, 1958 I 5 Sheets-Sheet 1 IN VE NFOR Henri JaLMd'eR-deBellescizq ATTORNEY MQ ZQ EET I 2 2,284,266

. H. J. J.'M.' DE REGNAULD DE BELLESCIZE SYSTEM FOR SIGNALING BY ELECTROMAGNETIC WAVES Filed April 7, 1938 5 Sheets-Sheet 2 lNVE/VI'OR Henri JJJW. defidefiellesczke BY (W ATT 01w: Y

May 26, 1942. 2 2,284,266

H. J. J. M. DE REGNAULD DE BELLESCIZE SYSTEM FOR SIGNALING BY ELECTROMAGNETIC WAVES Filed April 7, 1938 5 Sheets-Sheet 3 I INVENTOR I Henrz'cIdM. dgRdeBeZlescz'ze Mai 2a, 1942. v 4 2,284,266

H. J. J. M. DE REGNAULD DE BELLESCIZE v SYSTEM FOR SIGNALING BY ELECTHOMAGNETIQWAVES Filed April '7, 1938 5 Sheets-Sheet 4 v Q Q in; .2

E E M Q.

/N [/5 N TOR Henri J 4.McieR.deBellescz'ze ATTORNE Y May 26, 1942. 2,284,266

H. J. J. M. DE BEGNAULD DE BELLESCIZE SYSQP'EM FOR SIGNALING BY ELECTROMAGNETIC WAVES 7 Filed April 7, 1938 5 Sheets-Sheet 5 N Mm w/wa IIHM/ued WWW/5) Arms/V 5). I

Patented May 26 1942 '1 SYSTEM non SIGNALING BY ELECTROMAG- NETIC WAVES Henri Jean Joseph Marie de Regnauld de Bellescize, Neuilly-sur-Seine, France Application April 7, 1938, Serial No. 200,612 7 Claims. (61. 2.' 308) The invention concerns methods and systems of communication by electromagnetic waves, and chiefly a general method for neutralizing accidental fluctuations in the frequencies of signal Waves, this method being available even-when signaling is performed by frequency modulation without carrier or pilot wave. Such signaling occurs for instance in facsimile or code telegraphy, when each shade of the picture or each sign'of the code is represented by a particular value of the wave frequency; this system is well known, having formerly been practised with Poulsen arcs, and many recent technical developments have brought its advantages to light-: it affords the signal greater protection against interference of any kind; it allows of easier correction for general fading, for the wave amplitude at the transmitter remains constant and the signal is correctly reproduced even when'the detected current almost entirely vanishes; it also weakens selective fading, inasmuch as a larger number of important components is contained in the available part of the signal spectrum; furthermore, the weaker components remote from the centre of the spectrum are practically suppressed, and with them the interference they cause in adjacent signal channels; this last re sult is due to the fact that keying on the signal would be inoperative, since obviously they cannot discern Whether a rapid frequency change is accidental or due to signaling; furthermore, these cies successively used are not distributed about a constant mean value; for if a frequency F1f correspondsto the whiteparts of the picture and a frequency F1+f to the black ones, the mean of the frequencies actually used during anygiven time is not F1, but depends upon the mean shade of the element of picture transmitted during this time. v

The main object of the invention is toimprove methods and systems of communication by subwave frequency does not suddenly change either the amplitude or the phase of the wave.

The above advantages would be chiefly noticeable in short wave communication; but unhappily, the high frequency sources at the transmitter and at the receiver are liable to fortuitous frequency fluctuations, which up to now have prevented the detection of frequency changes representing the signal. This dificulty is very serious: for telegraphy by frequency changes is really promising only if these changes are not too large, say :150 czs; in which case errors of the order of ten cycles become relatively important; but H. F. oscillators usually suffer progressive frequency drifts amounting to several thousand cycles, and sudden leaps which may also exceed a thousand cycles, due to mechanical changes or to impedance variations caused by the automatic gain control.

The frequency stabilizingmeans disclosed by the prior art are unable to overcome this difficulty. Quartz controlled H. F, "oscillators still undergo excessive drifts, and moreover their natural frequencies are fixed once for all, which does not suit some purposes. The methods hitherto invented for synchronizing the oscillator stantially neutralizing the accidental frequency leaps and drift of the waves, the frequencies or amplitudes of which are modulated or varied to convey the signal. This neutralization iseffected by the oint action of two stabilizing means, the first designed to weaken and slow down considerably all rapid or slow fortuitous fluctuations which may affect-the oscillations generated in the transmitter and in the receiver, While nevertheless allowing the systematic modulation required for the signaling -and/or the-changes involved in tuning adjustments, while the second stabilizing means are designed to efiectneutr-alization of the remaining drift and yconsist of slowly acting control systems opfirated by frequency-changes in the signal wave, including'the signal "modulation, thus rendering the aid 0f a carrier or pilot wave unnecessary.

This method is of very wide application, for obviously it enables stabilization of. the frequency of an amplitude-modulated signal wave; this is indeed far more easy,.since all fre'quencyvar'iations are then accidental. .Several examples of systems embodying the invention will be described below. The stabilizingmeans-designed to weaken and slow down the fortuitous frequency fluctuationa both rapid and .s'l'owgcould of course consist of quartz crystalsror tuning forks imposing their own natural frequencies on the master oscillator .of (the transmitter andon the H. F. heterodyne oscillator .of the receiver;

of a receiving set with that of the transmitter ,but such means is-too rigid for manyapplica tions of the invention. A preferred method is to replace the natural instability of each. H, F; oscillator the frequency of which must vary responsively to'signaling or tuning devices, by the much smaller instability of a local medium frequency circuit or generator of relatively low and adjustable natural frequency In, this circuit or 7 generator being used to control almost instantaneously the frequency relation of the signal or heterodyne wave with an auxiliary electromagnetic wave characterized by a very stable frequency F; the frequency of the controlled H. F.

oscillation is thus fixed, exactly or approximately,

at one of the values Fo+fu or Fo -fa, and it can be adjusted within the same limits as in; up to fo=l0 czs, and even more, it is very easy, to provide adjustable M. F. circuits or generators with substantially constant natural frequencies.

In the transmitter, the rapid control exerted on the master oscillator by the corresponding local M, F. circuit or generatormay be rigid or loose; in other words, it may suppress entirely, or only weaken, frequency variations originating in this oscillator; the first case, signalingvariations will be obtained by submitting the'natural fre-' quency in of the local M. F. generator to .the direct influence of the signaling device, a telegraph key for instance; in the second case, this to an auxiliary wave of frequency F0 in such a way that while the frequency of the signal wave' I with the pliancy required for tuning and various purposes. Multiplex transmission of amplitude or frequency modulated signals can be obtained by the aid of a single auxiliary frequency F0 for controlling simultaneously several master oscillators, through corresponding local circuits or generators the natural frequencies in, I'll, f"o of which fix the frequency relation of the radiated signal waves; such a signaling system cangive fairly secret communication, since the reproduction calls for special stabilizing and selecting means which do not exist in usual receivers. Instead of being assigned to the transmission of several signals, the radiated waves may all be assigned to a single signal, so as to hasten its transdevice may act on the master oscillator itself,

but its action must now be muchstronger than it would need to be without stabilizing control, and therefore it will override the accidents causing fortuitousleaps. V The slow acting frequency control system may be based on the fact that all the signs of a telegraph code enjoy practically the same chances of being used; for each frequency F1f representing one of these signs, therefore, a frequency F1+;f enjoying the same chance of use and symmetrical with reference to the centre F1 of the spectrum may be adopted to represent another sign; the theory of probabilities proves that the mean of thefrequencies occurring successively during a signal is then very near F1, provided it'is the mean of several thousand code signs; applying this theorem, the

slow acting control device should be given a time constant several-thousand times the duration of each sign. In another system, which does not call for symmetrical distribution of the signal frequencies, this frequency control device may have two thresholds, coming into action when the lowest signaling frequency F1- '-j tends 'to be-' come too low, and when the highest signaling frequency F1+f tends to become too high.

According to another feature of the invention, the frequency stabilizing means are so designed as to work at will, either with usual telegraph codes based on two signs respectively represented by F1f and F1+ ,or with improved codes based on more than two signs and frequencies. For instance, such a code may be composed .of three signs represented by F1 .f, F1 and F1+ thus giv ing 81 possiblecombinations of 4 signs, whereas the Baudot code allows only 32 combinations of 5 signs; this improvement enables more words'to be transmitted in a minute for a given band width of the spectrum. The possibilities of improving the codes, thatis of increasing the nume ber' of signs and therefore the number of. frequencies, is a directconsequenceof the present invention:- for the more the accidental frequency variations are neutralized, the less need the signal frequencies differ-from one another.

'A further'purpose of. the inventionconsists in linking the signal wave radiated by an emitter.

mission, or to improve its protection against inte'rference and selective fading.

Similar improvements are obtainable in receivers, by placing the frequency relation of the H. F. heterodyne oscillation with an auxiliary. wave under the rigid control of the adjustable and relatively low frequency of a local M. F. generator. Tuning adjustments imparted to this local frequency are automatically transferred to the heterodyne frequency; by supplying the tuning condenser of the generator with a "frequency graduation identical with that of the generator controlling the transmitter, precise changes may be imparted very rapidly to the length of the signal wave, so as to avoid adverse interference and escape radio-direction searching; this is an advantage for naval and military communication. In multiplex reception, a single stabilized H. F. heterodyne oscillation may lower simultaneously the frequencies of all the received waves, so as to adapt them to their corresponding band pass local M F. generator shields the heterodyne oscillation from the disturbing influence of changes which may occur in the receiver, such, for instance, as those caused by its automatic amplification control. j

According to the invention, transmitters and receivers are provided with means by which'they can be immediately converted from aplitude modulation to, frequency modulation signaling and vice versa; thereby making communication possible between systems using either of these methods of signaling.

The natural frequency in of the local circuit or generator which controls the frequency relation of an auxiliary wave with a H. F. oscillation to be stabilized on a frequency F1 m'ay,,without detriment to the stabilizing action, be chosen within a fairly wide range, say between 0 and 10 czs; the auxiliary frequency Fo may therefore have any value reasonably different from Fi between F110 czs and'F1+10 .c:s. Consequently, the invention makesit possible to derive a frequency'control from any. stable received wave whatever, the auxiliary wave supplying this control being a suitable harmonic of asource,ofwhich Furthermore, the control exerted by the another harmonic is kept in look synchronism with this received wave. A single master station can thus control, within its range of action, the frequencies of all the H. F. oscillators in use for the transmission or reception of frequency or amplitude modulated signals.

In order to simplify the receiver, the device which transforms the frequency variations of the signal wave into intensity variations of a current used for neutralizing the drift of this wave, may also feed the recording apparatus. Any transforming device whatever may be used. That mention by W. H. Eccles in Wireless Telegraphy and Telephony (p. 454) depends for its action on amplitude changes along the slopes of the resonance curves or response characteristics of resonators or filters; as sometimes proposed, balanced detectors acting in opposition on an indicator may be energized by a pair of resonators respectively tuned to the extreme frequencies of the signal wave. Another system, disclosed in the applicants U. S. A. Patent No. 2,040,980, acts responsively to a synchronizing current by means of which a local source is kept in locked synchronism with the received wave. A third system, disclosed for instance in the German Patent No. 509,946, filed on October 16, 1928, for stabilizing the frequency relation of an oscillator with a pilot wave of constant frequency, may also detect frequency fluctuations; it is based on the variations in phase difference of two currents generated by the received wave in an aperiodic circuit and in a resonator, respectively.

In the accompanying drawings: Figure 1 illustrates the bands of frequencies allotted at a transmitter and a receiver to the simultaneous signals of a multiplex communication.

Figure 2 shows a scheme for neutralizing accidental leaps and drift which may affect the frequency of a master oscillator, While this frequency is being varied in accordance with a signal.

Figures 3 and 4 explain the working of a means used for transforming frequency changes into corresponding changes in intensity of a current.

Figures 5 and 5a illustrate one transmitter of a multiplex system, in which signaling may consist in modulating or varying either the frequency or the amplitude of a signal wave; the frequency of this transmitter is freed from fortuitous changes and nevertheless can undergo those required for signaling or for tuning.

Figure 6 shows a receiving set constructed according to the principles applied in the transmitter, for reproducing amplitude or frequency modulated signals.

Figures '7 and 7a illustrate a system for neutralizing a frequency drift, by the means of electromechanical relays energized by the currents respectively issued from the detection of the maximum and minimum signaling frequencies.

Figure 8 is a different realization utilizing purely electrical means.

Figure 9 explains *the working of the device illustrated at Fig. 8.

Figure 10 represents a system according to which a stable received wave of some frequency may supplyan auxiliary wave for stabilizing an emitter or a receiver working on another frequency.

Figure 11 concerns a communication system utilizing two signals waves, the frequencies of which are stabilized and nevertheless varying responsively to signaling apparatus; these waves maybe used, either for carrying two independent signals separatelyreceived'by the means 'of two recorders, or for concurrently transmitting a single signal received either on a single recorder for correcting the selective fading, or on two recorders for avoiding interferences.

In Fig. 1, a1 designates the band of frequencies. assigned to a signal radiated on a wave 01, the frequency of which may assume in response to the signaling. various values comprised between F1f;and F1+f; other signals may be transmitted 'within the bands (12, 0.3. After passage through the frequency changers of the receiver, these signal waves are converted to final frequencies comprised between F-f and pass filters having the response characteristics b1, in, be. For successful signaling, accidental frequency leaps which would be indistinguishable from signal modulation must be neutralized and so must also the drifts which could cause interference by mixing components pertaining to different signals. This double neutralization must be ensured both at the transmitter and at the receiver.

Figure 2 illustrates a system for stabilizing the master oscillator I of a transmitter, while allowing its frequency to be changed by a signaling device; this device is indicated as a key Econtrolling the grid bias and therefore the anode resistance of a tube 3, and varying the natural frequency of the resonator 5, 6 by means of a coil 4 in the anode circuit coupled to the resonator; according to the position of the key, the frequency of the oscillator should assume, for instance, the definite values Fi-f, F1, F1+f; for this to happen, fortuitous frequency fluctuations must substantially be suppressed. This suppression is effected by the combined action of two frequency control systems, both dependent on a local circuit 1 the natural frequency In of which, adjusted by a condenser 8, stabilizes approximately the frequency difference between the master oscillator and an auxiliary wave of frequency F0. This Wave, received at the terminal S, may come from a local source 9 with piezo-electric control or from a distant station. One of the controls, exerted through the coil l 0, is almost instantaneous, and acts as a damper to weaken and slow considerably all frequency fluctuations, both sudden and gradual, undergone by the master oscillator; the action of the signaling device, which depends on the closeness of coupling of the coils 4, 5, must therefore be made much stronger to produce the systematic changes if for signaling, and override the known or unknown causes of fortuitous fluctuation. The other control, applied through the coil II, neutralizes the remaining drift; it is very slow and consequently insensitive to signaling. The details of it are as follows: the oscillations generated by the oscillators 'l and 9 respectively coact in a frequency changer tube l2 to produce in the anode circuit of this tube a beat oscillation, transferred by the damped circuit !3 to the grid of the amplifier tube [4 which feeds a transformer l5. The secondary circuit of this transformer, the middle point of which is connected to the coil IE, constitutes a resonator l tuned by the condenser 8; its damping may be adjusted by means of a resistor l1 its output feeds the grids of two detectors l8, I9. As illustrated in Fig. 3, the vector e and are respectively selected in band representing the amplitude of the oscillations supplied to one of these grids is the resultant of a vector representing the oscillations impressed on the coil l6 and of a vector 11 representing those generated in one-half of the resonator 1;

likewise, the amplitude of the oscillations. supplied. to the other detector is represented by the vector e, the resultant, of the vector 0 and of a vector d which corresponds tothe other half of the resonator. The vectors d, d are in phase opposition and are in quadrature with'the vector 0 when the difference in frequency of the oscillators I, 9 is equal to the natural frequency of the resonator, i. e. when the master oscillator has exactly the frequency CF1=Fo-fo F1=F0+f0; otherwise, the phase relation of c and dd differs from 90. The difference in length ee' of the vectors e, e is thus-a function of the master oscillator frequency, as illusacross the resistors 20, 2| by'thetwo rectified currents the difference of which is read on the differential milliammeter 22; this voltage 11, which governs the frequency controls, reaches the values +v1 and -221 (Fig. 4) for the signal frequencies F1-- and F1+j respectively. One of the controls, that of short time constant, op-

erates through a tube 23 the anode resistance of which varies with the voltage o, and by means of a coil Ill coupled with the resonator 5, 6, varies the frequency of the master oscillator; this control, weakening and'slowin'g all frequency fluctuations undergone by the oscillator, makes it necessary to tighten the coupling of the coils 4, 5, in order to increase the action of the'signaling apparatus 2, and thus the'signaling action is made predominant in comparison with fortuitous causes of fluctuations; the effect of the key 2 should be somewhat delayed by a circuit 24, 25, to allow for the fact that the operation of this first control is not quite instantaneous, owing to the small time constant of the resonator I. The second frequency control, transmitted by the amplifier tube 26, is to suppress drift, i. e. maintain the central frequency of the signal spectrum at the exact value F1=Fo-fo; the device illustrated in Fig. 2 is suitable for signaling systems in which the frequencies representing the code signs have the same chance of being used, and are symmetrical in pairs with respect tothe central frequency F1, which may or not be used; under these conditions, which are fulfilled for instance in telegraph codes using two kinds ofsi'gns respectively represented by F1 and'F1+f, it may be shown that, among N consecutive signs of a signal, the number of one kind is almost certainly comprised between 0.5N-1.5/ N' and 0.5N+1.5\/N. Therefore, provided that the effects respectively produced by the two kinds of signs and frequencies are opposite, which is the present case for the voltages +v1 and -m, the

The effect ofN consecutive signs may be made cumulative by the aid of a condenser 21 and a resistor 28 connected to constitute a circuit of time constant N, times, larger than the duration of each code sign; then if '01 is the voltage which would be observed in the condenser as the result of a succession of N identical signs, the voltage actually produced by any given signal cannot exceed by increasing N, this voltage may be reduced as nearly "as desired zero, the value corresponding to the central frequency F1 of the signal spectrum. To sum up, provided that the time constant of the circuit 21, 28 is several thousand times larger than the duration of a sign, the bias larger to the tube 26 will correspond practically to central spectrum frequency, the drift of which may thus be detected and corrected; with N=2500, the possible error affecting this correction cannot exceed in other words six per cent of the signaling variation f. Reckoning from the usual duration of telegraph signs, the above explanation points to time constants of at least 50 seconds which are obtainable with a good condenser 21 of large capacity. :The anode circuit of tube 26,

'the grid bias of which varies in dependence on the drift, energizes a relay 29 which is so adjusted by means of a rheostat 30 that its armature rests midway between the contacts 3|, 32,

when the voltage 12 is zero, and closes the circuit of oneor other contact when the voltage reaches a very small'value, for instance 0.05 '01, corresponding to a drift of 0.05 f. The closing of either contact will vary the grid bias of tube 33'," and therefore its anode resistance and, through the coil ll coupled with the resonator 5, -6, will correct thefrequency of the master oscillator I; owing to this correction, the drift will be neutralized, and the voltage 1) will again become zero, whereupon the armature of the relay-will resume itsneutral position and stop the correcting action. But the time constant of the circuit 21, 28 delays the action of the voltage '12 on the grid of tube 26, thus causing a very large time lag between the instant at which the requisite correcting voltage is really applied and the instant at whichthe relay feels the effect of it; during this time lag, the relay the frequency corrections by the means of a clock-operated rotating switch 36, the contact 3'! of which closes, say every fifteen seconds,

' this period being chosen to be an appreciable cumulative effect of the Nconsecutive signs is at least timessmaller than if the N signs were all alike.

fraction of the time constant 27, 28, so that the charge of the condenser 21 has time enough to vary in response to each correction before the next occurs; furthermore, the duration of each closing of the contact 31 is so chosen in relation to the coupling of coil ID as to cause the correction required which approximately reduces to zero the current read on the milliammeter 22. To sum up, the device neutralizing drift has two time constants operating in succession; the first large enough to detect drift independently of the frequency changes due to signaling, the second adapting the speed of correction to the preceding time-constant. It follows that the device can only correct slow drifts and must be associated with the rapid control which, through tube 23 and coil in, brings about not only the attenuation of frequency leaps undergone by the master oscillator,,but also the substantial slowing down of its drift. The switch 38, usually placed in the position 39, may be momentarily moved to position 40 to suppress the action of the relay on the tube 33 and allow undisturbed initial adjustment which consists in tuning the condenser 6 of the master oscillator,

until the current read on the milliammeter 22 is zero, and then adjusting the rheostat 30 until the milliammeter 4| shows the current corre-- sponding to the neutral position of the relay 29. Figure 5, the essential features of which are reproduced in Fig. 5a; illustrates another similar system for neutralizing fortuitous fluctuations of a master oscillator, and at the same time compelling the oscillator to follow signaling fluctuations; the method still consists in associating an almost instantaneous control for weakening and slowing down frequency leaps and drift of the signal wave, with a slower acting control insensitive to the signaling to neutralize the remaining drift. The frequency difference between the master oscillator l and an auxiliary wave of frequency F0, supplied for instance by a crystal controlled oscillator 9, is now kept exactly equal to the relatively low natural frequency'fo of an auxiliary generator 42:! and 9 together supply a frequency changer i2 and produce a beat current which, combined in a second frequency changer 43 with an oscillation issuing from the generator 42, generates a synchronizing current for controlling the frequency of the oscillator I; this control, exerted by means of a coil 44, keeps in locked synchronism the two oscillations combined in 43; thus the frequency of the master oscillator is made exactly Fo+fo or Fo-fo; in order to impart to this oscillator the frequency changes if, the signaling apparatus 2 must now influence the natural frequency f of the generator 42'. In this system accidental fluctuations of the H. F. oscillator I are merely replaced by much smaller and slower fluctuations in a M. F.

generator42; furthermore, f0 being adjustable,-

the frequency Fn+fo of the radiated wave may be chosen within a fairly wide range of frequencies. Describing this system in more detail, the frequency changer l2 receives through the conductor 45 an oscillation generated by the master oscillator l, and through the amplifier tube 46 an auxiliary oscillation issuing from the oscillator 9, the frequency F0 of which is stabilized for instance by a tuning fork or by a quartz crystal; the tube 46 prevents the oscillator I affecting the conductor 41, to which may be connected at 43 other similar devices for controlling the other master oscillators of a multiplex system. The beat current issuing from the frequency changer I2 is fed by the damped circuit l3 and a conductor 49 to the second frequency changer 43, which also receives through 56 a more powerful oscillation issuing from the local generator 42; the relatively low frequency in of this generator may be tuned by a condenser graduated to show directly the number of cycles change in the frequency Foifu resulting from its adjustment. In

the anode circuit of the frequency changer 43 flows the synchronizing current, read on the milliammeter 52, and causing in a resistor 53 voltage changes depending on the phase relation of the currents respectively carried by 49, 54. These voltage changes together with fixed bias from the source 54 are applied to the grid of the tube 55, so as to vary the anode resistance of this tube, and, through the coil 44, the frequency of the master oscillator I. As is well known, to

synchronize the currents combined in the frequency changer 43, the condenser '56 must be tuned so as to bring the frequency difference of oscillators I and 9 near to the natural frequency of the generator 42; when locked synchronism is obtained, the frequency of the master oscillator becomes exactly Foifo, and then the only apparent result of further adjustment of the condenser 56 consists in changes in the current read onthe milliammeter 52; this current may .thus be restored to the value which would be observed,- by momentarily opening the switch 51, under the action of the generator 42 alone. The maximum amplitude of frequency correction caused by the synchronizing current is adjusted so that it may amply neutralize all accidental fluctuations, both leaps and drift, of the oscillator I;' if for in-' stance they reach kczs, the coupling of the coil 44 is adjusted to allow a maximum correction of kc:s; thus these fluctuations are suppressed and replaced by the much smaller fluctuations of the generator 42. The signaling device must now act on this generator was to impart to its frequency the variations if representing the signal; this device comprises the relays 58, 59, which control the grid bias of a tube through a switch 8| placed in the position 62; according as one or the other of the relays, or neither of them, is energized, the bias reaches one of the values corresponding to the tappings 63, 64 or 65 on a source of bias potential; the changes ensuing in the anode resistance of the tube affect the generator 42 through the coil 66, thus causing frequency variations which may be observed on the milliammeter 22, fed by the network I 4, l6, 1, I8,

19, already described for transforming frequency variations into current variations. Furthermore, should the small drift of 42 be. deemed still excessive, it could be neutralized by the slowly acting control system 25, 29, 35, 35, 34, 33, H, working as previouslyexplained. The master oscillator I, the frequency Fo-I-fc of which is now compelled to follow the fluctuations caused by keying in the frequency f0 of the generator 42', acts through the amplifiers 61, 68 and 69, 10 on the antenna H. Amplitude modulation signaling is obtained by merely throwing over the switch 6|,

whereupon the key 2 varies the amplification in tube Bl. Multiplex transmission may also be carried out by connecting at 12 another master oscillator, stabilized in the same way by an auxiliary wave issuing at 48 from the quartz controlled oscillator 9; and so on; the frequency relations of the various radiated waves are fixed by the natural frequencies in, fo, f"o of corresponding local generators and resonators, such as those illustrated at 42 and 1.

Instead of synchronizing the generator 42 with a beat current issuing from the coaction of H. F. oscillators I and 9, the auxiliary wave of frequency F0 could be combined in a frequency changer with an oscillation of frequency f0 supplied by the local M. F. generator 42; the beat component of frequency F0f0 could then be selected and utilized as a signal wave. The

6 Y master oscillator would then be the combination of the stabilized H. F. oscillator 9 with the M. F.

generator 42 the frequency, of which ,may be varied in accordance with a signal, or to change the wavelength of the transmission. To produce,

' must be much lower than F0, if itis to be pracdifiicult to separate the beat components Fpfo, F and Fo-Ho in the output of the frequency changer; this is why the system of Fig. is usually preferable.

Figure 6 shows areceiver in which the signal wave frequency is stabilized so as to be kept within the band width of a selecting filter. The method employed still consists in associating a very quick control for weakening and slowing down. frequency leaps of an adjustable H. F. heterodyne oscillation, with a slow acting control for neutralizing the remaining drift. The signal wave the frequency of which is varied at the transmitter between F1f and F1+f1, according to the signal, is received by the aerial I3, amplified at I4, and combined in a frequency changer tube I5 with an oscillation supplied by the heterodyne oscillator I6. The frequency difference between the heterodyne wave and an auxiliary wave of frequency F'o' issuing from a stable H. F. oscillator 11 is rigidly controlled by the local generator I8 the natural frequency f'o of which is relatively low and adjustable by means of a condenser 83: the oscillators'Ifi, 'I'I coact on a frequency changer I9, the latter directly, the former through the amplifier tube 80 which prevents the oscillator 'II affecting the receiver; the beat current generated by this coaction combines therefore be 7 rather tically free of drift andraccidental leaps; it will and by the aid of a M. F.- heterodyne oscillator 90, its frequency is again reduced in a frequency changer 9|; the finalfrequency of the signal wave, varying now from F-j to F-I-f, is chosen low enough, for instance near kczs, to facilitate theconstruction-of a filter 92 the band-width b1 (Fig. 1) of which must be somewhat wider than the total change 2f caused by signaling. This filteris followed by an amplifier 93 and then by a device 94 for stabilizing the amplitude of the received wave; this device-may be a limite r,or an automatic volume control, orboth. The signal thus amplified and stabilized is applied tofthe tube 95, the outputjof which supplies a network 'whichtransforms frequency changes into changes of current; the network illustrated comprises, as in the preceding figures, a coil 95- connectedwith the midpoint of a resonator 91, which feedsrectifiers 98, 99; the condenser I00 is tuned to make the natural frequency of the resonator coincide with the centre F of the band passed by the filter 92; thus the voltage difference 12 obtained between the points "ll, 2 is +01, 0, or --121, according as the final frequency in the signal channel is- Ff, F, or F+f; the variations of v energize on the one hand the indicator I03 and on the other hand the slow acting control which neutralizes the remaining drift. The response of indicator I03- is governedon the voltage 12 through the potentiometer I04, the switch I05 placed at I06, the amplifier tube I0'I the potentiometer I04 is adjusted so that the in another frequency changer 8| with a current supplied-by the generator'IB through the'amplifier 82; the low frequency current in the output of 81 causes voltage Variations across'the resistor 83, depending on the phase relation of the currents combined in 8|, and these control the heterodyne frequency through a conductor 84; when the condenser 85 is suitably tuned, the currents in 81 fall into locked synchronism, and this result is revealed by the milliammeter 86; the width of the band over which synchronizing action extends is adjusted by the potentiometer 81 so as to exceed the sum of the fortuitous fluctuations, both leaps and drift, to which the frequency of the heterodyne oscillator I6 may be subject; this frequency, thus perfectly fixed at F'o-l-f'o, becomes quite insensitive to any disturbing influence, for instance that of the gain control circuits, and yet may be varied together with f'o by adjusting the condenser 88; this adjustment,

substituted for that of '85, is by far more accurate since a given frequency alteration requires with the M. F. generator I8 a much larger change of capacity than with the H. F. oscillator I6; the condenser 88 may be graduated in cycles similarly to the corresponding condenser 5I (Fig. 5) on which the frequency of the transmitter depends; thus, agreed modifications may be rapidly imparted to the length of the signal wave; to sum up, for the properties of an adjustable oscillator have been substituted those of an adjust.- able M. F. generator, naturally much more stable. The M. F. oscillation produced in the frequency changer I5 by the beating of the signal and heterodyne waves is amplified and selected at 89,

' central frequency of the signal spectrum. The.

reading of the milliammeter I09, for 12:0, is the current needed to bring the armature of the relay midway between the contacts III), III. The slow acting control is the same as described for the transmitter; the changes occurring in the voltage 0 feed the amplifier H2 through potentiometer H3 and the circuit H4, H5 the time constant of which is several thousand times larger than the duration of a code sign; thus the changes'in the grid bias of tube H2 correspond approximately to the drift undergone by the potentiometer H3 is adjusted so that, for 12:0, the armature of the relay H6 rest midway between the contacts H1, H8, and the relay is made sensitive enough to close one or other of the contacts for a very small change in u, say +0.05 v1. The fluctuations caused in the grid bias of tube H9 by its connection to one or the other of the tappings I20, I21 are suitably delayed both by the timeconstant of a circuit I22,

I23- and by the operation of a rotating switch I24. Finally, the anode circuit of tube H9 controls through a coil I25 the natural frequency of one.

of the M. F. oscillators I8 or 90 of the receiver. To enable the receiver to reproduce amplitude modulated signals, it is only necessary to place the, switch I05 in the position I26; the grid bias.

of tube I0! is then constant, while thecathode voltage is reduced by each marking wave received by the detector 99; in other words, each marking wave raises the grid voltage of the tube I0'I with respect to the cathode, thus energizing the relay I08; the action is stable and exact, since the slowly acting control neutralizes the drift of the signal wave even more accurately when the frequency of this wave is independent of the signaling; the fluctuations of voltage 1) indeed, preserve automatically perfect coincidence of this frequency with the natural frequency F of the resonator 91, thus keeping in quadrature the vectors 0, d (Fig. 1) which represent the oscillations in the 'coil 98 and in the one half of resoand the relay I08;

nator 91 energizing the detector 99; the successive marking waves reach therefore the same resultant amplitude e and-produce rectified currents of like value. Multiplex reception of other signals may be simultaneously had by connecting in parallel with the frequency changer SI other frequency changers such as 9'5, each of them feeding a filter such as 92a, provided that the respective bands b1, b2 (Fig. 1) passed by these filters have the same frequency relations as the corresponding signal spectra :11, a2 radiated by the transmitter, the fact that the signal wave of frequency varying between F-f and F+f is kept within the band width of filter 92 ensures automatically accurate selection of the other received waves.

Figure 7 illustrates the neutralization of drift by means of a control system which does not require symmetrical distribution about the centre F of the spectrum of frequencies which have equal chances of being used; it operates only outside of two thresholds, under the action of the extreme signal frequencies Ff and F-l-f respectively, when the higher of them tends to increase or the lower to decrease. The tube I21 receives from a source I28, which may represent a combination of oscillators located both at the transmitter and at the receiver, a wave the frequency of which varies responsively to a signaling device I29; these frequency variations are transformed into corresponding changes in a voltage 12, for instance by the means already disclosed: namely a coil I30, a resonator I3I tuned to the central frequency F, of the signal spectrum, and

rectifiers I32, I33, feedingresistors I34, I together with a differential milliammeter I36. The voltage 1) varies from +111 for the limit frequency F-f, to 111 for F+J; its fluctuations, amplified by the tube I37, energize the .relay I38 which, instead of being designed to respond to very small variations of c, as previously described, closes the contact I30 when 7J=+D1, and the contact I 40 when v=-v1; thus as long as the wave frequency remains between its normal limits, the circuits controlled by therelay are open, and the charge of the condenser I4! is constant; but if the frequency representing a code sign happens to exceed these limits, the appropriate contact closes, charging or discharging the condenser through the resistor I 42 and changing the grid bias and anode resistance of tube I43, and, through the coil I44, the natural frequency of the source I23. The rela must now be rapid enough to follow the code signs, or at least the series of like signs Which occur at random from time to time during any transmission. Though more rapid than that illustrated by Figures 2 and 5, this drift neutralization must still be associated with a quasi-instantaneous system, for instance the local generator 42 of Figure 5, for neutralizing separately accidental leaps of frequency which .would be indistinguishable from signals.

As illustrated at Fig. 7a, the single relay I38 can be replaced by a pair of relays I38s, I381), one of them having an abutment I45& and a contact I 39 which closes for v +v1, the other having an abutment I45! and a contact I40 which closes for v v1; this allows of faster operation, each armature now making its full movement for very small changes of current; moreover, the thresholds 1171 may be separately adapted to any width of signal spectrum by adjusting the currents in the auxiliary windings I4I I4'Ib by means of the rheostats Mtg, I451).

In Figure 8, the thresholds of drift neutraliza- .purely' electrical means. I28 is, as above the tion actuated by the extreme frequencies F--f and F+fof the signal spectrum are defined by source of a signal wave influenced by the signaling apparatus I29. A device I48, I34, I35 transforms the frequency variations of the wave into voltage variations 1;, the extreme values of which are m and +111. These voltage changes are transferred to the grid of a tube I49, biased through I so as to place the threshold of its anode characteristic at v=+v;; thus the potential of the output tap I5I is constant for v +v1, and decreases in proportion as v rises above +01. The changes of v are also applied to a tube I52 working 'on a linear characteristic, which inverts them, the potential of the tapping I53 increasing when 11 decreases, and conversely; I53 is connected through a voltage source I54 to the grid of a tube I55, and the potentiometer I56 is ad- J'usted so as to place the threshold of this tube at v='u1; the potential of the output tapping I51 is therefore constant for v c1 and decreases in proportion as 0 falls below 01. Thence it follows that the voltage difference it between the tappings I5I, I51 is a function of v as shown in Fig. 9. Through a circuit I58, I59, of suitable time constant, the variations of u are impressed on the grid of a tube I60, thus influencing the anode resistance of this tube and the frequency of the source I28. This control operates only outside of the thresholds -01 and +01, passed over When the signal wave frequency exceeds its normal limits. stituted for the tubes I49, I;

One of the main features of the above systems consists in controlling, in transmission as well as in reception, the frequency relations of H. F. oscillators with auxiliary stable waves, by means of local M. F. generators, the frequencies of which may be changed responsively to tuning and/or signaling devices; furthermore, the

mean of the frequencies successively imparted to these generators may be controlled by adjustable circuits, such as that illustrated in Fig. 5,

in which a resonator I stabilizes the mean of the frequencies imparted by a key 2 to the generator 42. This method amounts merely to replacing the instability of adjustable H. F. oscillators by the far less dangerous instability of M. F. circuits or generators; the latter may easily reach a precision of about one in ten thousand or 0.01%, so that a natural frequency fo- 10 c:s will not be subject to accidental changes exceeding 10 c:s; such changes correspond to the precision of one in a million usually required for short waves. Thus for stabilizing with an accuracy of 0.0001 per cent the frequency F1 of a H. F. oscillation, it is sufiicient to secure some stable auxiliary wave of frequency F0 lying between F1-10 c:s and F1+10 c:s; this is always possible by starting from any stable received wave of frequency F5, maintaining a suitable harmonic m of a local source in locked synchronism with this wave, and utilizing another harmonic n of the source as the required auxiliary wave F0; in the resulting relation 771 the possible values of F0 differ by steps of which may be made smaller than 2.10 c:s by

Rectifiers of any kind could be subincreasing m; one of these values. will there fore be convenient, as difiering by less 10 czs from that frequency F1 Thus a single stabilized master station, illustrated at lfi'lin Fig; 10, could control the'H. F;

oscillators in all :receivers and transmitters lo- I cated near enough to receive a wave .radiated from this station; the system generating the auxiliary wave of frequency Fo' will comprise a receiver Hi2- collecting the wave of the master than v to be controlled' station, a sourceofalternating current l63, a

harmonic producer H54 supplying a suitable har-,

monic m of this source, and a frequency changer I65 for combining thisha-rmonic with the receivedwave, so as to keep harmonic and wave in locked synchronism by means of a synchr0 nizing current controlling thefrequency of the source through a connection I66; the auxiliary wave of frequency F0 arriving at theterminal Sof Figures 2, 5, 6 will be a harmonicnof the j source I63 derived from the harmonic producer I6'|.- Then, -as previously described, the local M. F. generator 42,,ofnatural frequency f0 varying responsively to signaling and/or tuning devices, controls the frequency relationof this a iliary wave with the wave of frequency Fo-fo 'which is either the signal wave radiated by a transmitter or theH. F. heterodyne wave of a receiver. This control could consist in merely combining in a frequency changer I68 the waves issuing from S and 42 respectively, and in seaesaaee. a

graph; codes ofv two. or three signs, respectively represented by thewave frequencies F-f, F+f, and may be F, errors caused by selective fading are avoidedby using a method based on two characteristics of the receiver illustrated in Fig. 6-: on the one hand, the accurate coincidence maintained by the drift neutralization between the central frequency F of the signal spectrum and the natural frequency, of the resonator 91 leads to the frequencyF-f being translated into a voltage +121, the frequency F-l-f into 2. voltage --'01, and the central frequency F into a zero w voltage; but the voltage observed in case of deep fading is also zero; therefore, if the marks +01 or m appear on; the tape of the recorder, they prove that the code sign represented by one of the'frequencies F-f or F+f has really been transmitted. On the other hand, two waves of unlike frequencies seldom suffer simultaneous selective "fading; thus, one at least of the marks proceeding at-the same moment from the waves respectively selected by the filters 92, 92a, is probably correct. 'From the above characteristics, it follows that, when the two tapes together admit of the mark corresponding to 12:0, this mark is probably right and the code sign represented by the frequency F has really been translecting'at K59 the beat oscillation of frequency Fo-f0. Many schemes based on thesame idea could of course be substituted for that shown in Fig. 10; for instance, a first source of alternating current could be synchronized with the received wave of frequency F5, and a harmonic p of this first source synchronized with a harmonic m of a second source of which another harmonic 11 would be the auxiliary wave offrequency F0; whence g r FF F,

What matters is the possibility of starting from a stable received wave of any frequency F5 tostabilize another frequency F1, while allowing this frequency F1 to follow the changes required for signaling and/or for tuning adjustment; the method consists in deriving from F5, through the synchronization of suitable harmonics of F5 and of local sources, an auxiliary frequency Fo=xF8, in choosing the integral or fractional number a: so as to obtain a difference low enough to be controlled by a naturally stable local generator or circuit, and in imparting to the natural frequency of this circuit or generator the changes required for signaling and/or for tuning adjustment. a V

The possibility l of maintaining within precise limits the successive frequencies imparted to a signal wave, so that several like waves may be used in juxtaposition within the narrow band of frequencies allotted to a station, has other applications than the multiplex transmission of several signals; such waves may also concur in carrying a single signal, either to hasten its transmission, or to improve its defence against interference or selective fading. For instance, the sending of facsimiles will be hastened by simultaneously scanning several lines of the picture, each of them controlling the frequency or amplitude of a corresponding wave. With telemitted; whereas unlike marks +111 (or m) and '0 denote the transmission of the code sign cor.-

responding to. the frequency Ff (or F+f). Thus, by comparing at the receiver two records of the same signal transmitted by simultaneously and similarly modifying the frequencies of two waves, an accurate text can'be reconstituted, provided that the drift is neutralized, and that the code includes two frequencies symmetrical with. respect to the natural frequency F of the resonator 91, with or without this natural frequency asa third. However, this method would hardly be practicable, and the same results can be reached automatically by means of a single recorder responsive to the joint action of the two waves. Figure 11 illustrates a communication system based on these principles. The left hand part of the figure represents a transmitter similar to that of Fig. 5; a M. F. local generator 42, the natural frequency ,fo of which is controlled by a key 2, controls rigidly the'frequency relation of the master oscillator l with an auxiliary wave of frequency F0 supplied by the stabilized oscillator 9;this control is effected by combining in a first frequency changer l2 two waves issuing from I and 9 respectively, and then in a second frequency changer 43 making an oscillation supplied by 42 heat with the beat oscillation issuing from l2; the synchronizing current collected from the output of 43 controls through a circuit 44 the frequency of the master oscillator, so that the oscillations combined in 43 are kept in locked synchronism; thus the frequency of the signal wave becomes Fofo, and varies responsively to the key 2; this wave is amplified at B1, 69, and radiated'by the aerial ll. Another like system Ia 2a. I28 423 43a. 60a. supplies a second signal wave. A systematic difference, for instance of 500 czs, is establishedbetween the mean values about which the natural frequencies ,fo, (rm of right hand part of the figure illustrates a receiver similar to that of Fig. 6; the two waves radiated by the transmitter are collected by an aerial 73, amplified at T L'and. combined in a frequency changer 15 with a heterodyne oscillation supplied by the H. F. oscillator 76; the difi'erence between the heterodyne frequency and the frequency Fo of a stabilized auxiliary oscillator 71 is rigidly controlled by the M. F. generator 78 of natural frequency fo; this control as before depends on a synchronizing current obtained by combining in 8| oscillations from the generator 18 with the beat current of oscillators 15, F7, coacting at 19; the heterodyne frequency, thus fixed at the value F'o-f'o may be modified together with ,f'c. After their passage through the frequency changer 15, which may be followed by other frequency changers, the two signal waves are respectively selected in the band pass filters 92, 92a, the bands b bz (Fig. 1) of which are given the same frequency relations as the spectra 1% radiated by the transmitter; Drift is neutralized by one of the systems of Figures 6, 7 or 8, comprising for instance a device ''I- (96, 91, 98,-99 of Fig. 6) for transforming frequency changes into voltage changes '11, and a device D (I I4, H5, H2, H6, H9) of large time constant for controlling through a coil I25 the frequency f'o of the generator 18; the action of this network can be suspended momentarily by opening the switch 33. The voltage 12 also energizes the amplifier tube I01 and a relay I08, designed to close one of the contacts H or I ll according as the code signs received are represented by the frequency Ff or F+f of the wave selected at 92, and to assume mid position for the voltage 11:0, whether due to the code sign represented by F, or to very deep fading of the signal wave. Likewise, the voltage changes Ua, caused at the output of the device Ta by the frequency changes of the other wave, act

on the tube 01a and relay "38a. The switch I13 may be placed at I74 for feeding separately the recording apparatus I03, 13 or at I15 for connecting the relays to the recorder I03 in parallel; the position I14 allows of the reception of two distinct signals; the position I! is adopted when the received waves are modulated at the transmitter by a single signal, in which case the operations of relays I08, Ififia are identical as long as the propagation is good. As selective fading may happen that one of them fades away, thus bringing the corresponding relay to its neutral position, but at the same time the other relay will work correctly and ensure an accurate recording. Therefore, this system of communication' makes it possible either in normal conditions to operate multiplex tansmission, or when there is deep fading to assign several waves to a single signal to improve its reception on a single recorder. According to a third method, the switch H0 of the transmitter may be placed at H2 so as to radiate two waves modulated by the same signal, and the switch I13 of the receiver at I14 so as to obtaintwo distinct recordings of this signal; should one of the received waves be accidentally disturbed by an adverse transmission, the other would probably remain free of interference, thus safeguarding the communication. It must be observed that strong interference coinciding with the wave selected by the filter 92 may paralyze the devices T, D, which neutralize drift; in that case the switch 38 must seldom affects the two Waves simultaneously, it a while f 9 be opened and drift momentarily corrected by handgaccordin'g to the indications of the differential milliammeter of the undisturbed recep- 'tion;

this operationis very easy and required only at long intervals, since the speed ofthe remaining drift is greatly lessened by the local generators '42 of the transmitter and 13 of the receiver. a

What I claim is; y,

-1. In a communication system utilizing a plurality of electromagnetic signal waves, a method of substantially lessening both in amplitude-and speed accidental changes undergone by the frequency of each ofsaid electromagnetic-waves, allowing of independent systematic changes in each frequency, which comprises providing an auxiliary wave of stable frequency relatively close to the frequencies of the electromagnetic waves, separately generating a local oscillations of natural fre'quencies tuned to the frequency diiferences of the said auxiliary stable and said first mentioned electromagnetic waves and therefore relatively'low, controlling these differences by means of the local oscillations,,and imparting the intended systematic changes to the natural frequencies of these oscillations. x1

2. In a communication system a method of substantially lessening both' in amplitude and speed accidental changes undergone by the frequency ofan electromagnetic wave, while allowing of systematic changes in this frequency, which comprise supplying an auxiliary wave of stabilized frequency relativelyclose to the frequency of the electromagnetic wave, deriving a beat component from the coaction of the said waves, separately generating a local oscillation of relatively low frequency tuned to this beat component, deriving. a synchronizing current from the coaction of this beat component with the local oscillation, controlling the frequency of the electromagnetic wave by means of this synchronizing current so as to keep the said beat component and local oscillation in locked synchronism, imparting the intended systematic changes to the natural frequency ofthis oscillation, and utilizing said wave of stabilized frequency for neutralizing accidental changes in the frequencies of a plurality of waves independently of said first mentioned frequency.

3. In a communication system, a transmitter, means for stabilizing the frequency of a signal wave While allowing systematic changes in the frequency and in the amplitude of this wave, said means comprising means for supplying an auxiliary wave of stable frequency relatively close to the intended signal frequency, a medium frequency generator supplying a local oscillation, means whereby the frequency of this oscillation controls the frequency relation of the signal wave and auxiliary wave, adjustable means for tuning the medium frequency generator within assigned limits of frequency, and means for utilizing said auxiliary wave of stabilized frequency for neutralizing accidental changes in the frequencies of a plurality of waves independently of said first mentioned frequency.

4. In communication system a receiver, means for stabilizing a highheterodyne frequency while permitting adjustment of it, said means comprising means for supplying an auxiliary wave of stable frequency relatively heterodyne frequencies, erator supplying a local close to the intended a medium frequency genoscillation, means whereplurality of 1 frequency relation of; the l ieterodyne and auxilia ave adju table means avi ituning t -medium frequency generator ,--within' as signed v -limits of frequency,-and means-for utilizing said iaanriliary wave of stabilized frequency; for neuti al-izing accidental changes in; the frequencies of a pluralityof waves independently-ofjsaid first mentioned frequency. j V V 5. In multiplex transmissionraflby :zirequency modulation of a pl urality, iofs sienakw v sha y low. natural frequencies tuned to the intended frequency difference :between; one; of :the a signal wavesand the auxiliary 'wave'yqmeans: for. placring :the; frequency relation: of.- each :signal wave with-the auxiliary'waveunder' the controlof the corresponding,localhoscillation, a plurality of sigggnaling-capparatus, ands switches; having two posi- :tions, in ;one, of which they; connect ,the several gsignalingapparatustogcontrol"the natural fre-V em {f r st liz n thcqfr q encyi :rclations ,of v r these-waves, which cqmprisesgsupplyingaone auxlliary electromagnetic-wave, of stablefreqixency 'relatiyely close to the frequencies assigned to the signal waves separatelygenerating, a plurality;

of localoscillations of ;reative1y low: naturalifrequencies, each; of said locali zosjcillations correspending to, one 10f the; signalgwaves. and. tuned 7 to the intended frequency difference between this wave;and.the auxiliary wavegcausing eachlocaloscillation to control-the frequencytrelation of.

- the correspondinggisignal-i wave 1 with the. auxilri 'y -.-wave-,-.: andj effecting. frequency modulation f0nasigna1ing;:by:. varying the: natural frequency j of:.each: local-oscillation ,inhaccordance with said signaling.

. 6. A'frequency modulation systeml for' -transmitting-by a plurality of cstabilizedsignal -waves either :seyeral .signals or in mnfavourable conditions asingle signal,-. which: comprises, a source .7 f or: supplying an rauxiliaryx wave of--stable -frequ'ency; relatively closeto -.the' frequencies as- I 1 signed to theisignal waves, a:plurality of-generators for supplying local oscillationsofrelatively quencies of the. respective local;v generators each dndependently of the others-in: accordance with q a signal, and in the otherof which they connect one,signaling;apparatus-to control the saidlocal generators simultaneously in accordance with the gsametssi nal.

7. A communicationsystemfor utilizing a plu- Y rality, of frequencyimodulated signal waves which comprisesrcontrolmeansifor neutralizing fortuitousiechangesin' the frequencies of the waves,

' meansqat the transmitter; adapted to the said control means fol-separately imparting to these 7 v frequencies systematic: changes in: ;correspondencewith: thesignaling; means at the receiver fortgseparately Z demodulating: :these systematic changes, and--switchesz botlrat the transmitter and; at, the receivers-for ialternatelyx.allowing. of

1 :two; operations, 1 the lOneZfOiJ affecting reachiiwave to {convey a r1signal',;:the: other dor'affecting 1.sev-

- ;jera-l-; waves tom-convey the.-;same:,signal,; whereby the waves; mayyberused sfor multiplex signaling in' normal conditionssanctrfor arsingle" signaling inunfavourable conditions,

HENRI TJEAN JOSEPH MARIE" DEZ'REGNAULD- DE BELLESCIZE. 

